1. Field of the Invention
The present invention relates to night vision apparatus and more particularly to a method and apparatus for producing a superimposed image of desired symbology on the view being seen in the night vision system.
2. Description of the Prior Art
Night vision systems are well known in the art. For example, the U.S. Government's Aviator's Night Vision System (ANVIS) goggle comprises a binocular arrangement containing apparatus for image intensifying low intensity light which is normally available from starlight or moonlight on a scene being viewed.
Apparatus for superimposing information on a scene being viewed through an optical system is also known in the art. Helmet mounted displays such as found in U.S. Pat. Nos. 5,113,177 and 5,299,063 utilize a partially silvered area on the helmet visor upon which information or symbology from a CRT is projected so as to be superimposed on the scene being viewed by the wearer. Binocular night vision systems having superimposed images thereon are also known as is shown in U.S. Pat. Nos. 4,915,487 and 5,079,416. In all of these systems a partially silvered mirror or prism is placed in the line of vision of the system so that information from the CRT can be reflected therefrom and combined with the view being seen by the wearer.
FIG. 1, although not specifically showing a prior art device, nevertheless shows a way that symbology could be introduced into the scene being viewed by a night vision system using conventional thinking. In FIG. 1, one ocular of the ANVIS night vision goggles, is shown by reference numeral 10. This device receives light from a dimly lit scene over a path shown by arrows 12 through a partially silvered mirror 14. The ANVIS contains image intensifiers which operate to increase the intensity of light received from the dimly lit scene. To inject desired symbology on this scene a cathode ray tube 20 is shown clamped to the ANVIS 10 by clamping member 22. Light from the CRT 20 passes generally along an axis shown by dash dot line 23 through a reflecting prism 24 and along and axis shown as dash dot line 25 through collimating lenses 26 and 28 to the partially silvered mirror 14 where the light from the CRT 20, shown by three sets of rays 29a, 29b and 29c, are then combined with the light passing through mirror 14 from arrows 12 so that the viewer sees both images simultaneously (only a few representative ray paths have been shown for simplicity). A CRT focusing ring 30 is utilized to cause the image seen by the viewer to be in focus. Connections such as 32 and 34 may be utilized to provide power and input information to the CRT in well known fashion.
Several difficulties arise with respect to the prior art and the apparatus of FIG. 1. A first problem arises because ANVIS 10 intensifies light which it receives and normally this light is very dim as received over paths such as shown by arrows 12. The light from the CRT, on the other hand, is much brighter than the dimly scene and accordingly the symbology which is displayed must be diminished to prevent the image intensifier tube in ANVIS 10 from being damaged. Of course filters or intensity modifiers can be utilized to reduce the amount of light from the CRT but such additional apparatus increases the weight, size and complexity of the system.
Another problem lies in the fact that the partially silvered mirror 14 allows only part of the light along paths such as arrows 12 to be received by ANVIS 10. As mentioned this light is already very dim and losing part of it makes the losses even greater in a very undesirable fashion and unbalances the intensities between the two oculars.
A third problem arises because the ANVIS night vision goggle has an adjustable focus so that it may view objects from far away and near. When the goggle changes it's focus, the image produced by the CRT may move out of focus. An additional adjustment of the CRT focus ring 30 will then be necessary.